Intrathecal Immunoglobulin Synthesis in MS—A Complete Reappraisal

Multiple sclerosis (MS) is characterized by the intrathecal synthesis (ITS) of immunoglo‐ bulins (Igs), which, although nonspecific, is the strongest biological marker. Since no specific target has been elucidated, this synthesis is considered to be disease‐irrelevant. We demonstrate that this synthesis provides pertinent information about the pathophy‐ siological processes involved. Quantification of ITS is based on an approximation intrinsically underestimating its level and it remains constant in MS, albeit sometimes at a low level. B‐cell maturation seems to be initiated within the cervical lymph nodes and B‐cells traffic on both sides of the blood‐brain barrier by rounds of bidirectional traffic. During this process, they undergo somatic hypermutation, which is the hallmark of antigen‐driven antibody maturation, suggesting that most of the ITS is probably directed against as yet unknown targets. Alternatively, examining”non‐disease‐relevant” ITS in the light of meningeal tertiary lymphoid organs provides new insights into the patho‐ physiology of MS. Although no specific target has yet been identified in MS, recent developments in the search for targeted antigens point to non‐conventional antigens (posttranslationally modified proteins or oxidized products) of which a few are promising for future research.